Good design, materials, and construction make
structures stable and strong
· Stability and strength depends on a structure’s
material as well as the fasteners used (bolts, welds, wire, thread, glue, etc.)
·
Stability: the ability of a structure to maintain or resume its
position when an external force has been applied to it.
Structural Strength
· Some structures have stood for thousands of years e. g.,
the Coliseum in Rome, the Pyramids in Egypt
Structural Shapes
· Some structural strength comes from the shapes used in
its design
· Triangles are stronger than squares and rectangles
· Triangular prisms are stronger than square and
rectangular prisms
Structural Components
· Arches, beams and columns are common structural
components that are used often because they can add strength and are attractive
· The components can be used alone or in combination
e.g., arches and columns
Structural Materials
· It is important to choose appropriate materials when
designing and building structures; designers should consider strength,
attractiveness, cost, etc.
Centre of Gravity
· Centre of gravity: the point at which a body’s mass is
concentrated – the body is equally balanced in all directions at this point
For example, when you
balance a ruler on your finger, the centre of gravity is the middle of the
ruler because each side of the ruler is symmetrical
· Every structure has a centre of gravity; the location
of the centre of gravity helps determine how stable the structure is
For example, a stool is a stable structure; however
when a person sits on the stool, the centre of gravity is higher so the stool
is more likely to tip over
Stability
· Stability depends on materials, construction
techniques and centre of gravity
E.g., a table can
have a high centre of gravity, but it can be stable if it has four legs far
apart
· Form can also affect stability; a solid structure with
a high centre of gravity may be less stable than a frame is
· Some structures are designed to be unstable; e.g.,
front ends of cars are meant to collapse easily in a collision
When Things Go Wrong
Structural Stress and Fatigue
· Poorly built structures may not be able to withstand
forces
· Large internal and external forces may weaken the
structure
· This can result in structural
stress
· A bend in a shelf is an example of this stress; the
shelf may go back to its original shape when the load is removed
· Permanent changes occur when the shelf cannot
withstand the stress; e.g., cracking. This is called structural fatigue.
Structural Failure
· Ignoring structural fatigue can lead to structural failure; this is the
breakdown of a structure due to the internal and external forces acting on it
· Structures often show signs of structural fatigue by
bending and cracking before finally failing and collapsing.
Product Recalls
· Public recall of seriously flawed products sold to
consumers by manufacturers
· Examples:
·
high
levels of paint in children’s toys
·
choking
hazards in products for children
·
overheating
batteries, poor safety features
·
cars
with faulty parts or design
